Just to be complete I should perhaps mention that a pull-up or pull-down is only needed when the signal source is only able to drive one way (a button switch can only connect to one of the power rails for instance. Most signal sources drive up and down (this is called totem-pole or push-pull output) - certainly most digital ICs drive most outputs this way.
Some buses and devices use an open-drain (sometimes called open-collector) output which only pulls down, requiring a pull-up resistor on the signal line. The I2C bus is a notable example.